1. Laboratoire de Glaciologie et Géophysique de l'Environnement (UMR CNRS/ Université Joseph Fourier 5183), 54, rue Molière, BP 96, 38402 St Martin d'Heres cedex, France
2. Department of Environmental Sciences, University of Venice, Ca' Foscari, 30123 Venice, Italy
3. Institute for the Dynamics of Environmental Processes-CNR, University of Venice, Ca' Foscari, 30123 Venice, Italy
4. School of Environmental Sciences, University of East Anglia, Norwich NR47TJ, UK
5. Korea Polar Research Institute, Korea Ocean Research & Development Institute, Ansan, PO Box 29, Seoul 425-600, Korea
6. École Polytechnique Universitaire de Grenoble (Institut Universitaire de France), Université Joseph Fourier, 28 avenue Benoît Frachon, BP 53, 38041 Grenoble, France
7. Max Planck Institute for Chemistry, Atmospheric Division, Joh.-J.-Becher-Weg 27, 55128 Mainz, Germany
8. Unité de Formation et de Recherche de Physique et Observatoire des Sciences de l'Univers (Institut Universitaire de France), Université Joseph Fourier, Domaine Universitaire, BP 68, 38041, Grenoble, France

Correspondence to: Carlo Barbante^2,3 Email: barbante@unive.it

Abstract

An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago¹. It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate² or the nanophase material of the vaporized impactor³. But the fate of platinum-group elements (such as iridium) that regularly enter the atmosphere via ablating meteoroids remains largely unknown. Here we report a record of iridium and platinum fluxes on a climatic-cycle timescale, back to 128,000 years ago, from a Greenland ice core⁴. We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age. We suggest that nanometre-sized meteoric smoke particles^{5, 6}, formed from the recondensation of ablated meteoroids in the atmosphere at altitudes >70 kilometres, are transported into the winter polar vortices by the mesospheric meridional circulation⁷ and are preferentially deposited in the polar ice caps. This implies an average global fallout of 14 5 kilotons per year of meteoric smoke during the Holocene.

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